1. Field of the Invention
This invention relates to the recovery of iron of relatively high metallic content by direct reduction of iron ores, concentrating the resulting sponge iron by grinding and separating it from the gangue and other waste material. The reduction operations is carried out at relatively high temperatures and under conditions both during and after reduction whereby carbon presence in the sponge iron is minimized. The sponge iron is recovered as iron flake powder after grinding and separation. Conventional apparatus may be used in the process. The process is particularly useful for high phosphorous iron ores.
2. Description of the Prior Art
Many processes have been proposed for the direct reduction of iron ores to produce metallic iron or the like. These processes are carried out at various temperatures ranging from below 900.degree. C. up to and over 1200.degree. C. and using various types of reducing agents. Generaly speaking, to effect complete reduction in a reasonable time the reduction must be carried out between 950.degree. C. and 1000.degree. C. the product may begin to sinter and at 1200.degree. C. a pasty porous mass may form. At 1300.degree. C. the mass absorbs carbon rapidly and begins to fuse. The upper melting point of the iron, that is the point at which it becomes liquid, is 1530.degree. C. Also at high temperatures, the ore will be reduced and carbon is absorbed from the fuel. Sulfur is also readily absorbed by the metal and is difficult to remove.
The processes which have been used for direct reduction have included well known types of apparatus, such as shaft furnaces, rotary and stationary kilns, retort furnaces, electric furnaces, fluidized bed reactors and many other types of furnaces. Reducing agents of all kinds have been used, such as coal, coke, graphite, char, distillation residues, fuel oil, tar, coal gas and hydrogen.
None of the foregoing processes, whether utilizing rotary kilns, shaft furnaces, fluidized beds, retorts or other equipment, suggest direct reduction of iron ores at temperatures from 900.degree. C. to 1200.degree. C. and thereafter handling the sponge iron so produced so as to minimize carbon at below 0.25% and preferably below about 0.10%, to recover metallic iron flake powder that is readily separated from gangue and other waste materials.
Typical among the prior art processes are those disclosed in U.S. Pat. No. 2,994,884 to Halvorson and U.S. Pat. No. 2,986,460 to Babcock et al.
Halvorson is to a method of producing high iron concentrates from iron ore by reducing and grinding wherein the iron ore is first crushed to -3/8 inch and finer and then mixed with carbon of a reactive form in an amount at least 50% more than theoretically needed for complete reduction of the iron ore to metallic iron. Although the furnace charge is heated to about 870.degree. C., and preferably to between about 1030.degree. C. and to about 1100.degree. C., such heating is for a prolonged period of time (about 18 to 21 hours). Also, it is carried out under conditions in which the iron is not only fully reduced, but it also carburized to produce iron carbide particles that lose malleability on fast cooling. Thereafter the charge is ground and the carburized iron particles contain at least about 0.65% carbon, an amount far in excess of that specified herein.
The Babcock et al patent is to a process of producing a sponge iron by direct reduction in a rotary kiln from iron ore having substantial gangue content wherein the ore is intermixed with a carbon reducing material and the reduction is effected at temperatures about 900.degree. C. The essence of the invention lies in the controlled cooling of such product under non-oxidizing conditions to obtain a sponge iron that is capable of being readily briquetted so as to serve as a feed for open hearth furnaces. However, minimizing of carbon is not important and the combined carbon may rise above 0.2% and often reaches values of 0.6% to 0.8% depending upon the metallization of the iron.